This invention relates to a radio communication apparatus and, more particularly, to a radio communication apparatus in a radio packet communication system for feeding back radio link quality information, which has been measured on a packet receiving side, to a transmitting side and adaptively controlling the modulation scheme and/or encoding rate on the transmitting side using the quality information.
In a radio packet communication system that uses adaptive modulation/demodulation and adaptive encoding such as High-Speed Downlink Packet Access (HSDPA) or High Data Rate (HDR), reception quality is measured from an already known downlink pilot symbol and is reported to the base station. On the basis of the reported value, the base station changes the encoding rate and modulation scheme (a combination of encoding rate and modulation scheme shall be referred to below as a modulation and coding scheme, or “MCS”) and then transmits the downlink packet, thereby improving throughput. If the MCS increases, the size of the information data transmitted increases but, on the other hand, so does the reception error rate. From the standpoint of throughput that takes retransmission control into consideration, use is made of adaptive MCS that conforms to reception quality. Further, the general practice is to implement ARQ (Automatic Repeat Request) by applying error detection encoding such as a CRC (Cyclic Redundancy Check) to a transmit packet, detecting reception error at the mobile station and feeding back the result to the base station as an ACK (Acknowledge) signal. Further, since the maximum number of data items transmitted simultaneously to a plurality of mobile stations is limited at the base station, a function for allocating transmission opportunity periodically to the transmit data of a specific one or a plurality of mobile stations becomes necessary. This function is referred to as “scheduling”.
A prior-art scheme relating to MCS selection involves exercising control in such a manner that the reception error rate of a mobile station is rendered constant. In regard to this prior art, two cases are conceivable, namely one where control is performed by the mobile station [see Michiharu Nakamura, Yassin Awad, Sunil Vadgama, “Adaptive Control of Link Adaptation for High-Speed Downlink Packet Access (HSDPA) in W-CDMA”, WPMC'02, October 2002] and one where control is performed by the base station [Toshiyuki Uehara, Makis Kasapidis, Katsuhiko Hiramatsu and Osamu Katoh, “One Study Relating to a Transmission Rate Allocation Method in HSDPA Base Stations”, Technical Report of IEICE, SST2001-77, A-P2001-225, RCS2001-260 MoMuC2001-57, MW2001-195 (2002-03)]. In a case where control is performed by the mobile station, error detection is carried out using CRC or the like and control exercised so as to lower the reported reception quality if an error is detected and raise the reported reception quality if an error is not detected, whereby the reception error rate converges to a constant value. In a case where control is performed by the base station, on the other hand, the ACK signal fed back by the mobile station is discriminated and control is exercised so as to enlarge the transmit. MCS (enlarge the data size of the transmit information) in case of ACK and diminish the transmit MCS (reduce the data size of the transmit information) in case of NACK, whereby the reception error rate converges to a constant value. A stabilized packet error rate characteristic is obtained as a result. In addition, by setting a target error rate beforehand so as to maximize throughput, it is possible to stabilize and improve throughput.
FIG. 11 is a block diagram of a conventional radio packet communication system in which a base station adaptively controls a modulation scheme and/or encoding rate based upon a target error rate as well as radio downlink quality information and mobile-station reception success/failure information received from a mobile station.
A mobile station 1 has a receiver 1a for receiving a high-frequency signal sent to it from a radio base station 2, converting the signal to a baseband signal and inputting the baseband signal to a packet receiver 1b and pilot-symbol receiver 1c. The packet receiver 1b demodulates encoded packet data, applies decode processing and inputs the decoded signal to an error detector 1d. The latter performs error detection using an error detecting code (CRC code) contained in the decoded signal and inputs the result of detection to an uplink signal generating unit 1e. Meanwhile, the pilot-symbol receiver 1c extracts a pilot symbol and inputs the symbol to a reception quality measurement unit 1f. The latter measures the SIR (Signal to Interference Ratio) as the reception quality of the radio downlink using a known pilot symbol and the received pilot symbol, converts the measured SIR as indicated by Equation (1) below and inputs the result of the conversion to the uplink signal generating unit 1e as downlink reception quality information [referred to below as “CQI” (Channel Quality Indicator)].CQI=f(SIR)=[10×log10(SIR)+C]  (1)where C represents a fixed value given in advance and [X] the largest integer that will not exceed X.
The uplink signal generating unit 1e incorporates ACK/NACK, which is the result of error detection, and the downlink reception quality information CQI in the uplink signal, and a transmitter 1g subjects the uplink signal to modulation processing and frequency conversion and transmits the resultant signal. FIG. 12 illustrates an example of a data format for transmitting ACK/NACK and CQI on an uplink HS-DPCCH channel in 3GPP.
The base station 2 has a receiver 2a for receiving uplink signals from a plurality of mobile stations 1, applying a frequency conversion and demodulation processing to the receive signal and then separating ACK/NACK and CQI from the signal and outputting the same. The base station 2 has a scheduler 2b which, on the basis of the quality information CQI from each mobile station, selects the mobile station to which the next packet is to be transmitted and inputs this information to an MCS decision unit 2c and transmit-data buffer 2d. Conventional examples of schemes for selecting a mobile station that is to be the destination of a transmission include a scheme (Max CIR scheme) for selecting the mobile station that has the best reception quality at that moment, and a scheme (Proportional Fairness scheme) for selecting the mobile station having the largest ratio of momentary reception quality to average reception quality.
Next, using a CQI-MCS conversion table, the MCS decision unit 2c decides the MCS (modulation scheme and encoding rate) of the packet to be transmitted to the mobile station that has been selected by the scheduler 2b. It should be noted that the MCS decision unit 2c decides MCS upon referring to the conversion table after an offset, which is decided by ACK/NACK and the target error rate, is added to the received CQI. The offset decided by ACK/NACK and the target error rate is added on because there is only one conversion table and it is necessary to apply a correction so as to obtain a CQI value that conforms to the communication environment. In this case, if the value of the offset is updated in accordance with Equation (2) below based upon ACK/NACK of the mobile station, the reception quality of mobile station 1 can be made to converge to a target error rate (PER).
                    {                                                                            ⁢                                  Offset                  =                                      Offset                    +                                          α                      ⁢                                                                                          ⁢                      if                      ⁢                                                                                          ⁢                      ACK                      ⁢                                                                                          ⁢                      is                      ⁢                                                                                          ⁢                      received                                                                                                                                                              ⁢                                  Offset                  =                                      Offset                    -                                          β                      ⁢                                                                                          ⁢                      if                      ⁢                                                                                          ⁢                      NACK                      ⁢                                                                                          ⁢                      is                      ⁢                                                                                          ⁢                      received                                                                                                                              (        2        )            where α and β are related as follows: α/β=PER/(1-PER). For example, assume that the target error rate (PER) is 10−1 and that α=0.1, β=0.9 hold. If the error rate attains the target error rate (PER) in this case, ACK is received nine times out of ten and the offset is increased by a total of 0.9, NACK is received one time out of ten and the offset is decreased by 0.9, and the overall increase/decrease in the offset is zero, whereby convergence to a constant value is achieved. The reception quality CQI converges to a value conforming to the target error rate (PER). The MCS decision unit 2c decides and outputs the MCS corresponding to the CQI that conforms to the communication environment and target error rate.
It should be noted that an offset is retained separately for each mobile station and that offset updating is performed at the timing at which ACK/NACK is received even with regard to mobile stations other than the mobile station that has been selected by scheduling.
If the mobile station that is to transmit information and the MCS (modulation scheme and encoding rate) of each mobile station have thus been decided, the transmit data buffer 2d sends the decided transmit-information data of the mobile station to a transmit packet generator 2e in a size (=TBS) decided by the MCS. The transmit packet generator 2e has the structure shown in FIG. 13. Specifically, a channel encoder 2e-1 encodes the transmit data based upon an encoding rate (=TBS/n) using the transmit-data bit count (=TBS) and transmit-code bit count (=n) decided by the MCS decision unit 2c, and a modulator 2e-2 creates a downlink radio packet by performing digital modulation in accordance with the modulation scheme (QPSK/16-QAM) similarly decided by the MCS decision unit 2c. A transmitter 2f transmits this radio packet and a known pilot, which enters from a pilot generator (not shown).
It should be noted that in the case of CDMA, the downlink radio packet transmitted to each mobile station is spread by a channelization code conforming to the mobile station, after which the packet is combined and input to the transmitter 2f. Further, in a case where the packet is transmitted upon application of frequency multiplexing, the data is converted to frequency data conforming to the mobile station and then the frequency data is multiplexed and transmitted. A case where the transmit-code bit count (=n) is input to the transmit packet generator 2e has been described above. However, it can be so arranged that the encoding rate r is calculated by the MCS decision unit 2c and input to the transmit packet generator 2e. 
A radio packet transmitted from the mobile station 2 has been encoded to make error detection possible. The mobile station 1 feeds back an ACK symbol if an error has not been detected and a NACK symbol if an error has been detected. Further, the mobile station 1 measures the SIR based upon the pilot symbol, calculates the CQI and feeds it back to the base station. The prior-art communication system for radio packets achieves high-speed packet communication by repeating the above operation.
FIG. 14 is a block diagram of the MCS decision unit 2c. Specifically, an offset calculating unit 2c-1 calculates the offset for every mobile station according to Equation (2) based upon ACK/NACK, holds the offset and inputs it to a CQI correction unit 2c-2. The latter adds the offset to the reception CQI to thereby correct the CQI. In case of CDMA, a table 2c-3 stores transport block sizes TBS, code counts Ncode and modulation schemes (QPSK/16-QAM) in association with respective ones of CQIs, as shown in FIG. 15, and outputs a TBS, Ncode and modulation scheme that conform to the entered CQI. It should be noted that the smaller the CQI, i.e., the poorer the downlink reception quality, the smaller the transport block size TBS, the smaller the number of codes multiplexed and, moreover, the smaller the number of data items sent by a single modulation operation. As a result, the target error rate can be met even if the reception quality of the radio downlink varies.
A transmit-code bit count calculating unit 2c-4 calculates the transmit-code bit count n, in a manner described below, using Ncode and the modulation scheme, and outputs n. Specifically, since packet length Nchip is such that Nchip=7680 [chips]=0.2 ms holds and spreading factor SF is fixed at SF=16 [chips], the number M of bits that can be transmitted per packet is as follows:
In case of QPSK: Mbit=Nchip/SF×2=960 bits
In case of 16-QAM: Mbit=Nchip/SF×4=1920 bits
As a result, the transmit-code bit count n is given by the following:n=M×Ncode  (3)The transmit-code bit count calculating unit 2c-4 calculates the transmit-code bit count n in accordance with the above equation and outputs n. By way of example, if CQI=1 holds, then the transmit-code bit count n is equal to 960×1. It should be noted that it is also possible to calculate the encoding rate r in accordance with the following equation:r=TBS/(M×Ncode)  (4)and output the encoding rate r instead of the transmit-code bit count n. If CQI=1 holds, then the encoding rate r is equal to 137(960×1)=0.14.
FIG. 16 is a block diagram of another example of a conventional radio packet communication system in which components identical with those shown in FIG. 11 are designated by like reference characters. This example differs in that the mobile station 1 is provided with a reception-quality report value controller 1h. The latter corrects the offset based upon Equation (2) depending upon whether or not an error has been detected, corrects the reception quality information CQI, which enters from the reception quality measurement unit 1f, using the corrected offset, and transmits the corrected CQI and the ACK/NACK information to the base station 2 in accordance with the format of FIG. 12. The base station 2 performs retransmission control using the ACK/NACK information.
FIG. 17 is a timing chart of a radio packet communication system. Here one slot is defined as the reception quality measurement time, CQI transmit/receive time and packet transmit/receive time at the mobile station 1. The time interval from the moment the mobile station 1 measures reception quality at a certain timing and transmits the CQI to the moment the mobile station 1 transmits the CQI next shall be referred to as a CQI feedback cycle Tcycle. Further, the time interval from the moment the mobile station 1 measures reception quality to the moment a packet that reflects the result of measurement is actually received shall be referred to as the CQI delay time. The shortest CQI delay time shall be referred to as minimum CQI delay time Tmin, and the longest CQI delay time shall be referred to as maximum CQI delay time Tmax. In the timing chart of FIG. 17, the interval from reception quality measurement #0 to reception quality measurement #1 is Tcycle (=6 slots), the interval from reception quality measurement #0 to packet reception #00 is Tmin (=3 slots), and the interval from reception quality measurement #0 to packet reception #05 is Tmax (=8 slots). These values are values given beforehand from a higher order layer.
The target error rate that maximizes throughput differs depending upon the status of the mobile station. For example, the optimum target error rate in a case where throughput has been optimized under conditions where reception quality does not fluctuate from the moment the mobile station measures reception quality to the moment a packet is actually received differs greatly from a case where throughput has been optimized under conditions where reception quality fluctuates significantly owing to high-speed travel of the mobile station or a reception-quality reporting period that is too long. In the prior art, therefore, a problem which arises is that throughput declines under conditions where a target error rate set in advance is not always the optimum target error rate. For example, the prior art is such that if the fading frequency rises, errors increase and the offset becomes negative in accordance with Equation (2). As a result, the CQI for satisfying the target error rate decreases. This leads to a decrease in transmit data quantity and a decline in throughput.